AP Chapter 9

• Why is it easier to open the door from the outer edge instead of closer to the hinges?

• Your force is applied farther from the pivot point resulting in a larger lever arm.

• The lever arm combined with the force creates a new phenomena called torque

• τ = Fl τ is called tau and units are Nm

• What is the net torque on a 4 m teeter totter if you, 58 kg, and your friend, 45 kg, are on opposite ends with the fulcrum (pivot) in the center?

• τ =(58kg)(9.8m/s2)(2m) –(45kg)(9.8m/s2)(2m)

• τ = 255 Nm

0 m2m 2m

58 kg 45 kg

• In equilibrium

• ΣFx = 0 Σ Fy = 0 Σ τ = 0

• Steps to solve• 1) Free-body diagram• 2) Choose convenient axes• 3) Apply the force equations• 4) Choose convenient pivot point• 5) Apply torque equation• 6) Solve

• If you, 55 kg, climb a 6.0 m, 25 kg ladder and stand on the rung that is 5.6 m up and the ladder is 40o from the ground. How much force does the wall apply to the ladder assuming no friction with the wall? How about the ground?

Fw

Fyou

Flad

Fgy

Fgx

6.0m

5.6m

3.0m

40o

• ΣFx = Fgx – Fw = 0 Fgx = Fw

• ΣFy = Fgy – Flad – Fyou = 0• Fgy = Flad + Fyou = • Fgy = (25kg)(9.8m/s2) + (55kg)(9.8m/s2)• Fgy = 784 N

• Στ = Fw(6.0m)sin40o – Fyou(5.6m)sin50o – Flad(3.0m)sin50o = 0

• Fw(6.0m)sin40o= (55kg)(9.8m/s2)(5.6m) sin50o + (25kg)(9.8m/s2)(3.0m)

sin50o

• Fw = 750 N

• Fgx = Fw = 750 N